There is strong interest in solar energy as an alternative to fossil fuels. But solar cells, also known as photovoltaic (PV) cells, must generate electricity at a competitive cost to fossil fuels, to be commercially viable. To be competitive, PV cells must utilize low cost materials, be inexpensive to make, and have moderate to high conversion efficiency of sunlight to electricity. Moreover, all aspects of making the PV cells must be commercially scalable.
The photovoltaic market is presently dominated by silicon wafer-based solar cells (first-generation solar cells). The active layer in those solar cells utilizes single crystal silicon wafers having a thickness of microns to hundreds of microns. Silicon is a relatively poor absorber of light. Single-crystal silicon wafers are very expensive to produce because the process involves fabricating and slicing high-purity, single-crystal silicon ingots. The process is also very wasteful. For those reasons, much development work has focused on producing high efficiency thin film solar cells having material costs significantly lower than silicon.
Semiconductor materials like copper indium gallium diselenides and sulphides (Cu(In,Ga)(S,Se)2, herein referred to as “CIGS”) are strong light absorbers and have band gaps that match the optimal spectral range for PV applications. Furthermore, because those materials have strong absorption coefficients, the active layer only a few microns thick can be used in a solar cell.
Copper indium diselenide (CuInSe2) is one of the most promising candidates for thin film PV applications due to its unique structural and electrical properties. Its band gap of 1.0 eV is well matched with the solar spectrum. CuInSe2 solar cells can be made by selenisation of CuInS2 films. During the selenisation process, Se replaces S and the substitution creates volume expansion, which reduces void space and reproducibly leads to a high quality, dense CuInSe2 absorber layers. [Q. Guo, G. M. Ford, H. W. Hillhouse and R. Agrawal, Nano Lett., 2009, 9, 3060] Assuming complete replacement of S with Se, the resulting lattice volume expansion is ˜14.6%, calculated based on the lattice parameters of chalcopyrite (tetragonal) CuInS2 (a=5.52 Å, c=11.12 Å) and CuInSe2 (a=5.78 Å, c=11.62 Å). This means that the CuInS2 nanocrystal film can be easily converted to a predominantly selenide material, by annealing the film in a selenium-rich atmosphere. Therefore, CuInS2 can be used as a precursor for CuInSe2 or CuIn(S,Se)2 absorber layers.
The theoretical optimum band gap for solar absorber materials is in the region of 1.2-1.4 eV. By incorporating gallium into CuIn(S,Se)2 nanoparticles, the band gap can be manipulated such that, following selenization, a CuxInyGazSaSeb absorber layer is formed with an optimal band gap for solar absorption.
Conventionally, costly vapor phase or evaporation techniques (for example metal-organic chemical vapor deposition (MO-CVD), radio frequency (RF) sputtering, and flash evaporation) have been used to deposit the CIGS films on a substrate. While these techniques deliver high quality films, they are difficult and expensive to scale to larger-area deposition and higher process throughput. Thus, less expensive and more flexible methods of producing the component layers in PV cells are desirable.